DE69301418T2 - Double heat pump cryogenic rectification system - Google Patents

Description

Technical area

This invention relates generally to the cryogenic rectification of mixtures containing oxygen and nitrogen, e.g. air, and more particularly to a cryogenic rectification process and apparatus for producing high pressure product gas as defined in the preambles of claims 1 and 7, which are based on EP-A-0 042 676.

State of the art

The demand for high pressure oxygen gas is increasing due to the increased use of high pressure oxygen in partial oxidation processes such as coal gasification for power generation, hydrogen production and steel production. Nitrogen is also often used in these processes.

Oxygen gas is produced commercially in large quantities generally by cryogenic rectification of air. One way to produce the oxygen gas at high pressure is to compress the product oxygen gas from the cryogenic rectification unit. However, this is expensive both in terms of capital costs for the product oxygen compressor and in terms of operating costs in powering the product oxygen compressor. Another way to produce high pressure oxygen gas is to operate the cryogenic rectification unit at a higher pressure, thus producing the oxygen at a higher initial pressure and reducing or eliminating the downstream compression requirements. Unfortunately, operating the cryogenic rectification unit at a higher pressure reduces the efficiency of the production process because the separation of the components depends on the relative volatilities of the components, which decrease with increasing pressure. This is particularly the case when high pressure nitrogen product is also desired from the cryogenic rectification unit, since the withdrawal of nitrogen from the high pressure distillation column as product reduces the amount of reflux that can be used, thus reducing the oxygen yield.

A cryogenic rectification process for producing high pressure product in which:

(A) transition cooling an elevated pressure feed air stream and introducing the resulting feed air fluid into a high pressure column;

(B) feed air is separated in the column operating at high pressure by means of low temperature rectification into first nitrogen-rich fluid and oxygen-enriched fluid;

(C) introducing the first nitrogen-rich fluid and the oxygen-enriched fluid into a column operating at a lower pressure and separating them therein into the second nitrogen-rich fluid and the oxygen-rich fluid by means of cryogenic rectification;

(D) withdrawing a second nitrogen-rich fluid from the column operating at a lower pressure; and

(E) withdrawing oxygen-rich fluid from the column operating at a lower pressure, pressurizing the oxygen-rich fluid to a higher pressure, and recovering a transition-heated fluid as high pressure product oxygen;

is known from EP-A-0 042 676, which also describes a cryogenic rectification device for producing high pressure product with:

(A) a feed air compressor, a heat exchanger, a first column and an arrangement for passing feed air from the feed air compressor to the heat exchanger and from the heat exchanger to the first column;

(B) a second column and means for transferring fluid from the first column to the second column;

(C) a pump, means for transferring fluid from the second column to the pump and from the pump to the heat exchanger; and

(D) an arrangement for recovering fluid from the heat exchanger is disclosed.

In this known method and in this known device, an open nitrogen cooling circuit is provided above the head of the high-pressure column in order to generate an additional reflux for the high-pressure column.

It is an object of this invention to provide a cryogenic rectification system which can produce high pressure product gas with improved efficiency over results achievable with conventional systems, particularly when both oxygen and high pressure nitrogen product gas are desired.

Summary of the invention

The above and other objects which will become apparent to those skilled in the art from this disclosure are achieved by means of the present invention, one aspect of which is a cryogenic rectification process for producing high pressure product, in which:

(A) transition cooling an elevated pressure feed air stream and introducing the resulting feed air fluid into a high pressure column;

(B) feed air is separated into first nitrogen-rich fluid and oxygen-enriched fluid in the high pressure column by means of cryogenic rectification;

(C) introducing the first nitrogen-rich fluid and the oxygen-enriched fluid into a column operating at a lower pressure and separating them therein into the second nitrogen-rich fluid and the oxygen-rich fluid by means of cryogenic rectification;

(D) withdrawing a second nitrogen-rich fluid from the column operating at a lower pressure; and

(E) withdrawing oxygen-rich fluid from the column operating at a lower pressure, pressurizing the oxygen-rich fluid to a higher pressure, and recovering a transition-heated fluid as high pressure product oxygen;

characterized in that

- in process step (D) the second nitrogen-rich fluid is further compressed, the compressed second nitrogen-rich fluid is transition cooled, and the resulting second nitrogen-rich fluid is introduced into the high pressure column, and

- in process step (E), the pressurized oxygen-rich fluid is further cooled by means of indirect heat exchange with the transition-cooling system under increased pressure stagnant feed air and the transition-cooling, compressed, second nitrogen-rich fluid.

Another aspect of the invention is a cryogenic rectification device for producing high pressure product, comprising:

(A) a feed air compressor, a heat exchanger, a first column and an arrangement for passing feed air from the feed air compressor to the heat exchanger and from the heat exchanger to the first column;

(B) a second column and means for transferring fluid from the first column to the second column;

(C) a pump, means for transferring fluid from the second column to the pump and from the pump to the heat exchanger; and

(D) means for recovering fluid from the heat exchanger;

characterized in that the device is further provided with

(E) a nitrogen compressor, means for transferring fluid from the second column to the nitrogen compressor, from the nitrogen compressor to the heat exchanger and from the heat exchanger to the first column.

As used herein, the term "indirect heat exchange" refers to bringing two fluid streams into heat exchange relationship without any physical contact or mixing of the fluids with each other.

As used herein, the term "transition heating" refers to either heating a fluid resulting in its vaporization from the liquid state to the vapor state or heating a fluid at a pressure above its critical pressure through a temperature range including the critical temperature of the fluid.

As used herein, the term "transition cooling" means either cooling a fluid resulting in its condensation from the vapor state to the liquid state, or cooling a fluid at a pressure above its critical pressure from an initial temperature that is at least 1.2 times its critical temperature to a final temperature that is within the range of 0.5 to 1.1 times its critical temperature.

As used herein, the term "feed air" means a mixture containing primarily nitrogen and oxygen, such as air.

As used herein, the term "compressor" means a device for increasing the pressure of a gas.

As used herein, the term "expander" means a device used to extract work from a compressed gas by lowering its pressure.

As used herein, the term "column" refers to a distillation or fractionation column or zone, i.e., a contact column or zone in which liquid and vapor phases are contacted in countercurrent to effect separation of a fluid mixture, such as by contacting the vapor and liquid phases on vapor-liquid contact elements, such as a series of vertically spaced trays or plates disposed within the column and/or on packing elements, which may be structured and/or random packing elements. For further discussion of distillation columns, see Chemical Engineer's Handbook, Fifth Edition, edited by R.H. Perry and C.H. Chilton, McGraw-Hill Book Company, New York, Chapter 13 "Distillation," B.D. Smith et al., pages 13-3, The Continuous Distillation Process.

Vapor and liquid contact separation processes depend on the difference in vapor pressures for the components. The component with high vapor pressure (or the more volatile or low boiling component) tends to concentrate in the vapor phase, while the component with low vapor pressure (or less volatile or high boiling component) tends to concentrate in the liquid phase. Distillation is the separation process in which heating of a liquid mixture can be used to concentrate the volatile component(s) in the vapor phase and thus the less volatile component(s) in the liquid phase. Partial condensation is the separation process in which cooling of a vapor mixture can be used to concentrate the volatile component(s) in the vapor phase and thus the less volatile component(s) in the liquid phase. Rectification or continuous distillation is the separation process which combines successive partial evaporation and condensation steps as obtained by countercurrent treatment of the vapor and liquid phases. The countercurrent contacting of the vapor and liquid phases is adiabatic and may involve integral or differential contact between the phases. Separation process arrangements which use the principles of rectification to separate mixtures are often referred to interchangeably as rectification columns, distillation columns or fractionation columns. Cryogenic rectification is a rectification process which which is carried out at least partly at low temperatures, such as at temperatures at or below 150ºK.

As used herein, the terms "upper section" and "lower section" refer to those sections of a column above and below the center of the column, respectively.

Short description of the drawings

Fig. 1 is a schematic representation of a preferred embodiment of the cryogenic rectification system according to the invention.

Fig. 2 is a schematic representation of another preferred embodiment of the cryogenic rectification system according to the invention.

Detailed description

The invention generally comprises a dual heat pump arrangement in which oxygen pumped to high pressure, which may be at a pressure greater than its critical pressure, is transition heated against both transition cooling input air and transition cooling nitrogen. Preferably, the transition cooling feed air stream has between 25 and 75 percent of the transition cooling fluid stream in heat exchange with the transition heating oxygen. If only feed air were used to transition heat all of the oxygen product, the oxygen yield would be poor. If only nitrogen were used to transition heat all of the oxygen product, the resulting large flow of nitrogen reflux would exceed the reflux requirements needed to compensate for the poor yield, and furthermore, the necessary nitrogen compression would consume large amounts of energy. To optimize the system, at least a portion of the feed air is transition cooled at a temperature compatible with the temperature of the transition cooled nitrogen. The transition cooling of this feed air, in conjunction with the transition cooling of the nitrogen, provides the amount of heat required to transition heat the product oxygen to the desired pressure. The split between the feed air and nitrogen streams against the transition heating oxygen can be varied and optimized by balancing the lower pressure ratio feed air compressor output against the higher pressure ratio nitrogen compressor output and the base load air compressor output or return flow nitrogen compressor output, if used.

The invention will be described in detail with reference to the drawings. With reference to the drawings, feed air 100 is introduced by passing it through a Base load air compressor 1 to a pressure in the range of 4.1 to 31 bar (60 to 450 pounds per square inch absolute (psia)), preferably in the range of 8.3 to 31 bar (120 to 450 psia). Compressed feed air 101 is then passed through a purification system 2 to remove high boiling contaminants such as water vapor, carbon dioxide and hydrocarbons to produce purified feed air 110. A 10 to 50 percent portion 114 of the feed air is pressurized to an elevated pressure in the range of 8.3 to 207 bar (120 to 3000 psia), preferably in the range of 9.7 to 138 bar (140 to 2000 psia) by passing through a feed air compressor 3. The resulting feed air 115 at elevated pressure is cooled by indirect heat exchange in heat exchanger 5 against return streams, and resulting cooled feed air 16 at elevated pressure is transition cooled by passing through a heat exchanger 8. The resulting cooled feed air is passed into column 9. The embodiment illustrated in the drawings represents a particularly preferred embodiment in which transition cooled feed air 17 from heat exchanger 8 is expanded to the pressure of column 9 through a valve 102 and heated by passing through a subcooler 15. The resulting heated feed air 19 is then introduced into column 9.

Another portion 111 of the purified feed air 110 is cooled by passing it through heat exchanger 5, the resulting stream 112 is further cooled by passing it through heat exchanger 6, and the resulting cooled purified feed air 113 is introduced into column 9. Those skilled in the art will recognize that heat exchangers 6 and 8 may alternatively be combined into a single heat exchanger.

The first column or high pressure column 9 operates at a pressure in the range of 4.1 to 31 bar (60 to 450 psia). In the high pressure column 9, the feed air is separated into a first nitrogen-rich fluid and an oxygen-enriched fluid by cryogenic rectification. Oxygen-enriched fluid is withdrawn as a liquid from the lower section of the column 9 as stream 40 and cooled by passing through heat exchanger 13. The resulting stream 41 is passed through valve 103 and then introduced into column 11 as stream 42. First nitrogen-rich fluid is withdrawn as a vapor from the upper section of the column 9 as stream 104. A portion 105 of the first nitrogen-rich vapor is condensed in main condenser 10 by indirect heat exchange with boiling bottoms liquid from column 11. A first portion 106 of the resulting condensed nitrogen-rich fluid is returned as reflux to column 9. A second portion 70 of the resulting condensed nitrogen-rich fluid is cooled by passing it through heat exchanger 12. Resulting nitrogen-rich fluid 71 is passed through valve 107 and then introduced as stream 72 into column 11.

The second or lower pressure column 11 is operated at a pressure less than the pressure of column 9 and in the range of 2.1 to 7.6 bar (30 to 110 psia). In the lower pressure column 11, the feeds are separated into a second nitrogen-rich fluid and an oxygen-rich fluid by cryogenic rectification. Second nitrogen-rich fluid is withdrawn as vapor stream 80 from the upper section of column 11 and is heated by passing through heat exchangers 12 and 13 by indirect heat exchange with first nitrogen-rich fluid and oxygen-enriched fluid, respectively. The resulting second nitrogen-rich stream 81 is further heated by passage through heat exchangers 6 and 5 and removed from the system as stream 85 which can be recovered as product nitrogen gas having a purity of generally at least 95 percent and preferably at least 99 percent. A portion 86 of stream 81 withdrawn from the upper section of the lower pressure column or second column 11 is passed to a nitrogen compressor 4 as more fully described below.

A stream of first nitrogen-rich fluid is withdrawn from the upper section of column 9. This stream is shown as stream 50 which is part of stream 104. Stream 50 may optionally be withdrawn from main condenser 10, for example as part of liquid stream 106, pumped to a higher pressure and transition heated through heat exchanger 6 from which it exits as stream 51 as illustrated in Figure 2. As shown in the drawings, nitrogen-rich vapor 50 is heated by passing through heat exchanger 6 and exits heat exchanger 6 as stream 51. In the embodiments illustrated in the drawings, a portion of vapor stream 51 is passed as stream 52 through a nitrogen expander 7 in which it is expanded to a lower pressure to produce refrigeration. The majority of stream 51 is passed through heat exchanger 5 as stream 54 and then withdrawn from the system as stream 55 which is recovered as high pressure nitrogen gas having a purity of generally at least 99 percent and preferably at least 99.9 percent.

In the embodiments illustrated in the drawings, the expanded first nitrogen-rich vapor 53 exiting the nitrogen expander 7 is combined with stream 81 to form a combined stream 82 which is passed through the heat exchangers 6 and 5 as described above and exited from the system as stream 85. A portion of the expanded first nitrogen-rich vapor may also form a portion of the nitrogen stream 86.

Nitrogen-rich vapor stream 86 is pressurized by nitrogen compressor 4 to a pressure in the range of 8.3 to 207 bar (120 to 3000 psia), preferably in the range of 9.7 to 138 bar (140 to 2000 psia), and the resulting compressed stream 87 is cooled by passing through heat exchanger 5 to form cooled nitrogen-rich vapor stream 88 which is additionally transition cooled by passing through heat exchanger 8. Resulting nitrogen-rich fluid 89 is introduced as additional reflux into column 9. In the embodiments illustrated in the drawings, nitrogen-rich fluid 89 is additionally subcooled by subcooler 15 and the resulting subcooled stream 90 is passed through valve 108 and then introduced as stream 91 as reflux into column 9.

Oxygen-rich fluid is withdrawn as liquid stream 60 from the lower section of the lower pressure column and pressurized by pump 14 to a pressure in the range of 2.8 to 207 bar (40 to 3000 psia), preferably in the range of 2.8 to 138 bar (40 to 2000 psia). The resulting oxygen-rich fluid 61 is then passed through heat exchanger 8 in which it is transition heated by indirect heat exchange with transition-cooling feed air at elevated pressure and transition-cooling compressed nitrogen-rich fluid which contains the second nitrogen-rich fluid from the second column and may also contain the first nitrogen-rich fluid from the first column. The resulting transition-cooled oxygen-rich fluid 62 is further heated by passing it through heat exchanger 5 and recovered as product high-pressure oxygen gas 63 having a purity in the range of 70 to 99.9 percent, preferably in the range of 90 to 99.5 percent.

A computer simulation of the invention was performed using the embodiment of the invention as illustrated in Figure 1 and the results of this example are presented in Table I in which the stream numbers correspond to those of Figure 1. The example is presented for illustrative purposes and is not intended to be limiting. TABLE I Flow designation Normalized molar flow Pressure bar Temp (ºK)

By using the dual heat pump arrangement of this invention, in which high pressure oxygen-rich fluid is transition heated against both transition cooling feed air and transition cooling nitrogen, a cryogenic rectification plant can now be operated at higher pressures than conventionally used, while achieving improved yield efficiency over conventional plants operating at higher pressures than conventionally used.

Claims (9)

1.Cryogenic rectification process for producing high pressure product in which: (A) a feed air stream (110) under increased pressure is transition-cooled and the resulting feed air fluid (17, 19, 113) is introduced into a column (9) operating at high pressure; (B) feed air is separated in the column (9) operating at high pressure by means of low-temperature rectification into first nitrogen-rich fluid and oxygen-enriched fluid; (C) first nitrogen-rich fluid (72) and oxygen-enriched fluid (42) are introduced into a column (11) operating at a lower pressure and separated therein by means of cryogenic rectification into second nitrogen-rich fluid and oxygen-rich fluid; (D) withdrawing a second nitrogen-rich fluid (80, 86) from the column (11) operating at a lower pressure; and (E) withdrawing oxygen-rich fluid (60) from the column (11) operating at a lower pressure, pressurizing the oxygen-rich fluid to a higher pressure, and recovering a transition-heated fluid (63) as high-pressure product oxygen; characterized in that - in process step (D) the second nitrogen-rich fluid (86) is further compressed, the compressed second nitrogen-rich fluid (87) is transition cooled, and the resulting second nitrogen-rich fluid (91) is introduced into the high pressure column (9), and - in process step (E), the pressurized oxygen-rich fluid (61) is further heated by means of indirect heat exchange with the transition-cooling, pressurized feed air (115) and the transition-cooling, compressed, second nitrogen-rich fluid (87). 2. The method of claim 1, further comprising heating transition-cooled, elevated pressure feed air (18) by indirect heat exchange with the transition-cooled, compressed, second nitrogen-rich fluid (89) to produce the second nitrogen-rich fluid before the feed air fluid and the second nitrogen-rich fluid are introduced into the column (9) operating at high pressure. 3. A process according to claim 1, wherein the first nitrogen-rich fluid (104) is withdrawn from the upper part of the high pressure column (9) and recovered as nitrogen product. 4. The method of claim 3, wherein the first nitrogen-rich fluid (107) is liquefied, pressurized to a higher pressure and transition-heated before being recovered as a nitrogen product (55). 5. A process according to claim 1, wherein first nitrogen-rich fluid (52) is withdrawn from the upper part of the high pressure column (9), expanded to produce cold and heated by indirect heat exchange with feed (111, 112) which is then introduced into the high pressure column. 6. The process of claim 1 wherein the flow rate of the transition cooling feed air is 25 to 75 percent of the flow rate of the total transition cooling fluid in the heat exchange with transition heating oxygen-rich fluid. 7. Device for low-temperature rectification to produce high-pressure product with: (A) a feed air compressor (1, 3), a heat exchanger (5, 6, 8), a first column (9) and an arrangement for transferring feed air from the feed air compressor to the heat exchanger and from the heat exchanger to the first column; (B) a second column (11) and means for transferring fluid from the first column to the second column; (C) a pump (14), means for transferring fluid from the second column (11) to the pump and from the pump to the heat exchanger; and (D) means for recovering fluid from the heat exchanger (5, 6, 8); characterized in that the device is further provided with (E) a nitrogen compressor (4), an arrangement for transferring fluid from the second column (11) to the nitrogen compressor, from the nitrogen compressor to the heat exchanger (5, 6, 8) and from the heat exchanger to the first column (9). 8. Apparatus according to claim 7, further provided with a subcooling arrangement (15) through which both the arrangement for transferring fluid from the feed air compressor (1, 3) to the heat exchanger (5, 8) and to the first column (9) and the arrangement for transferring fluid from the nitrogen compressor (4) to the heat exchanger and the first column pass. 9. Apparatus according to claim 7, further comprising means for withdrawing and recovering fluid from the upper part of the first column (9).